The invention relates in general to the detection of the contact of a tool to a workpiece for further machining operations.
In precision machine operations, the point at which a tool contacts a workpiece must be accurately determined for additional tooling operations. Visual observation is inadequate in many situations because of the size and/or shape of the machine. Feeler gauges are usually not practical when the tool is moving.
Contact sensing systems are known in the prior art that electrically insulate a workpiece from the tool. The tool then completes an electrical circuit when it contacts the workpiece, which often produces a visible or audible effect. These systems are generally satisfactory when used with stationary, single-edge tools maintaining continuous contact with the workpiece, such as a drill. However, rotating multi-edge tools, such as a milling tool, used with such systems can create inaccuracies.
One such contact sensing system is presented in U.S. Pat. No. 3,476,013 to Zemberry. Zemberry teaches a contact sensing apparatus utilizing two transformers with each primary connected in series to a power source. When a tool and a workpiece make contact, the secondary of one transformer short circuits, resulting in an impedance change lighting a lamp connected across the secondary of the other transformer. The use of transformers in the sensing circuit can result in low sensitivity because leakage inductance attenuates the high frequency signal components caused by the short contact time of the tool. The use of AC excitation to detect the contact can also allow substantial contact before the bulb lights. Finally, such a design does not address changes in the conditions in which the sensing system is being used, such as the presence of lubricants, often used in tooling, which form an electrically conductive bridge between the workpiece and the machine base.
The present invention is a tool tip conductivity sensor and method for detecting the contact of a tool to a workpiece. The method of the present invention comprises the steps of electrically coupling a first lead to one of the workpiece and a holder supporting the workpiece and a table supporting the workpiece and the holder; electrically coupling a second lead to a machine operating the tool at a point electrically insulated from the first lead; generating an input signal whenever a circuit between the first lead and the second lead is closed; and generating an indicator signal responsive to the input signal indicating the contact of the tool to the workpiece.
In one aspect of the invention, the method further comprises the steps of comparing a DC offset to the input signal; generating a trigger signal from the trigger circuit whenever the input signal is greater than the DC offset; and generating the indicator signal in response to the trigger signal. In this aspect, the step of comparing the DC offset to the input signal preferably comprises the steps of supplying the input signal to a first input of a comparator and supplying the DC offset to a second input of the comparator. The trigger signal is the output of the comparator. The step of generating the indicator signal in response to the trigger signal can comprise the steps of supplying the trigger signal to a comparator; connecting a resistor to an input of the comparator; and connecting a capacitor in parallel with the resistor, the capacitor receiving a charge from the trigger signal and dissipating the charge across the resistor when the trigger signal ends. In this aspect, the output of the comparator is the indicator signal. The method can further include the step of generating the DC offset using a power supply supplying power to a resistor network.
In another aspect, the invention further comprises the step of supplying the indicator signal to at least one of an LED, an audible device, and a position signaling circuit supplying a position signal to a controller of the tool. In this aspect, the method can further comprise the steps of electrically coupling the first lead to a contactor of a contact probe; generating a reference position when a contact of the tool to the contactor closes the circuit between the first lead and the second lead; and comparing the position signal to the reference position for further machining operations.
In another aspect of the invention, the input signal is a pulsed trigger signal and the step of generating an indicator signal comprises the steps of supplying the pulsed trigger signal to a comparator; connecting a resistor to an input of the comparator; and connecting a capacitor in parallel with the resistor, the capacitor receiving a charge from the pulsed trigger signal and dissipating the charge across the resistor when the pulsed trigger signal ends. Then, the output of the comparator is the indicator signal. This aspect of the invention can further include the step of supplying the indicator signal to at least one of an LED, an audible device, and a position signaling circuit supplying a position signal to a controller of the tool. As mentioned previously, in this aspect, the method can further comprise the steps of electrically coupling the first lead to a contactor of a contact probe; generating a reference position when a contact of the tool to the contactor closes the circuit between the first lead and the second lead; and comparing the position signal to the reference position for further machining operations.
In a preferred aspect of the invention, the method further comprises the steps of supplying the indicator signal to a first oscillator adapted to received the indicator signal and supplying an activation signal from the first oscillator to a piezoelectric acoustic transducer. This aspect can further comprises the steps of supplying the indicator signal to a second oscillator, the second oscillator supplying an output to the first oscillator, and periodically preventing resonance of the piezoelectric acoustic transducer using the output of the second oscillator.
The apparatus of the present invention for detecting a contact of a tool to a workpiece comprises means for electrically coupling a first lead to one of the workpiece and a holder supporting the workpiece and a table supporting the workpiece and the holder; means for electrically coupling a second lead to a machine operating the tool at a point electrically insulated from the first lead; means for generating an input signal whenever a circuit between the first lead and the second lead is closed; and means for generating an indicator signal responsive to the input signal indicating the contact of the tool to the workpiece.
Preferably, the apparatus includes at least one of an LED, an audible device, and a position signaling circuit supplying a position signal to a controller of the tool. In this aspect, the apparatus can further comprise a contact probe including a contactor adapted to be electrically coupled to the first lead and carrying an electrical contact wherein a contact of the tool to the contactor closes the circuit between the first lead and the second lead to generate a reference position; and means for comparing the position signal to the reference position for further machining operations. In the aspect with an audible device, the device is preferably a piezoelectric acoustic transducer.
In one aspect of the invention, the means for generating an input signal whenever a circuit between the first lead and the second lead is closed comprises a trigger circuit comparing a DC offset to the input signal and generating a trigger signal whenever the input signal is greater than the DC offset. In this aspect, a power supply can supply power to a resistor network, which provides the DC offset. The trigger circuit can be a comparator wherein the input signal is supplied to a first input of the comparator and the DC offset is supplied to a second input of the comparator. Also in this aspect, the trigger signal can be supplied to a pulse stretching circuit, where the pulse stretching circuit includes a comparator adapted to receive the trigger signal; a resistor connected to an input of the comparator; and a capacitor connected in parallel with the resistor, the capacitor receiving a charge from the trigger signal and dissipating the charge across the resistor when the trigger signal ends. The output of the comparator is the indicator signal.
In another aspect of the invention, the apparatus further comprises a first oscillator adapted to receive the indicator signal and a piezoelectric acoustic transducer receiving an activation signal from the first oscillator. In this aspect, the apparatus can further include a second oscillator receiving the indicator signal and supplying an output to the first oscillator, wherein the output of the second oscillator periodically prevents resonance of the piezoelectric acoustic transducer.
In yet another aspect of the invention, the input signal is a pulsed trigger signal and the means for generating an indicator signal comprises a pulse stretching circuit includes a comparator adapted to receive the trigger signal; a resistor connected to an input of the comparator; and a capacitor connected in parallel with the resistor, the capacitor receiving a charge from the trigger signal and dissipating the charge across the resistor when the trigger signal ends. The output of the comparator is the indicator signal. This aspect has a number of variations. For example, the apparatus can further include at least one of an LED, an audible device, and a position signaling circuit supplying a position signal to a controller of the tool. In the aspect including an audible device, preferably the audible device is a piezoelectric acoustic transducer. Where a position signal is supplied, the apparatus further comprises a contact probe including a contactor electrically coupled to the first lead and carrying an electrical contact wherein a contact of the tool to the contactor closes the circuit between the first lead and the second lead to generate a reference position and means for comparing the position signal to the reference position for further machining operations. In another variation, the apparatus further comprises a first oscillator adapted to receive the indicator signal and a piezoelectric acoustic transducer receiving an activation signal from the first oscillator. In this aspect, a second oscillator receiving the indicator signal and supplying an output to the first oscillator can be included, where the output of the second oscillator periodically prevents resonance of the piezoelectric acoustic transducer.
Thus, the present invention is a highly sensitive and accurate tool tip conductivity sensor. It can accurately detect contacts under a variety of conditions. Since the sensor produces a signal upon each contact of the tool to a workpiece, the sensor can also be used to indicate tool position and to monitor for breakage of the tool.